This invention relates in general to imaging systems.
Ultrasonic systems have been used in medical imaging as well as other applications. Typically, pulse echo imaging techniques are used. In most common use are linear arrays utilized in systems constructed by Acuson, ATL, Diasonics, and others. These arrays are used to generate a narrowly focused waveform and utilize electronically controlled multi-element transducer arrays. These arrays have improved the ability to focus the ultrasonic pulse thus improving imaging accuracy. However, the arrays currently in use are limited to imaging in two dimensions.
Recently, two-dimensional arrays have been introduced that require additional elements to support an equivalent focusing capability in three dimensions. As the number of ultrasonic elements increases, so does the cost and difficulty of manufacture. All of these systems employ a pulse echo approach and are therefore limited by physical principles in their ability to focus their ultrasonic beam. These systems are unable to focus a pulse narrower or shorter than the ultrasonic wavelength employed by the systems. There is therefore a lower limit to the resolution of these systems. Employing ultrasound at higher frequencies is possible but is disadvantageous since the proportion of energy absorbed by the body in medical applications increases with the frequency of the ultrasound, thereby limiting imaging depth at high frequencies.
To alleviate the above-described difficulties in conventional pulse echo approaches, back propagation techniques have been proposed. In these techniques, a broad ultrasonic beam of pulses is transmitted and the times of arrival of the pulses at a number of receivers around the image region are then measured. Back propagation techniques do improve imaging accuracy beyond the limitations of traditional pulse echo systems by achieving imaging accuracy corresponding to fractional wavelength of the ultrasound. Back propagation systems, however, are limited by their physical configuration and are predominantly two-dimensional imaging devices. In order to image a three-dimensional volume, a large number of transducers would be required, thereby increasing cost and decreasing flexibility to the point when such systems may be impractical based on the current technology.
None of the above-described systems is entirely satisfactory. It is therefore desirable to provide an improved system for imaging a region where the above-described difficulties are not present.